Are you asking about how to do a basic necropsy at home, or where to send a bird to have it tested?
Calling a local poultry extension agent, your state vet, the closest teaching veterinary hospital, or your closest vet who sees birds- should get you a mailing address and forms to fill out to send in a bird to be tested for various infectious diseases. These necropsy reports usually also contain info on what parasites the chicken had, cultures for infectious agents, and specific damage to tissues, tumors ect. They do not in general screen for toxins, but damage to certain organs might be compatible with certain toxins. Cost is often the cost of postage.
If you are talking about doing a necropsy at home- all you are going to find is really obvious stuff like crop full of grass, large roundworms, egg yolk peritonitis, large tumors ect. You will not be able to do the fine detail things that need microscopes and tissue fixation, cultures ect. You also need to know normal anatomy to recognize abnormal anatomy. ie abnormal heart, liver, kidneys, thickened sciatic nerves from Marek's...
Doing home necropsies can be interesting, and sometimes give you an answer- but often the cause is not visible to the naked eye....
